A fluid refers to a substance that is capable of flowing. A thixotropic fluid is a fluid that flows under mechanical stress application and does not flow in the absence of a mechanical stress. As a consequence, a thixotropic fluid that has been shaped through flow in the presence of a mechanical stress essentially maintains its shape and dimensions after removal of the mechanical stress. This means that the shape and dimensions are essentially maintained in the absence of a container (i.e., a mold) for the fluid. The thixotropic character allows the making of objects of specified shapes and dimensions without using molds. An example of a thixotropic fluid is toothpaste.
Molds are containers that have the shape and size of the object to be fabricated. Fabrication of objects without using molds simplifies the fabrication process and reduces the cost of fabrication. Molds tend to be expensive and demolding (conducted after the completion of the molding) is an additional non-trivial step. The difficulty of demolding stems from the tendency for the molded material to adhere to the mold.
Three-dimensional (3D) printing refers to printing layer-by-layer in the absence of a mold to form a 3D structure (Silverbrook, US20140345521; Matsumoto, US20150005920; Heikkila and Heikkila, US20150080495). An example of a 3D structure is a gear. The 3D object resulting from the 3D printing in the context of this Specification is to be distinguished from a print product that is functionally two-dimensional (2D), though it appears 3D, as obtained, for example, by printing multiple layers of ink on a piece of paper. Such 2D products (though having a 3D appearance) cannot be detached from the substrate (such as a piece of paper) to form a standalone object. In contrast, this invention concerns 3D products that can be detached from the substrate to form a standalone object.
Because metals are involved in numerous parts (such as auto parts) and structures (such as car bodies), 3D printing of metals is technologically important. However, the printing of metals is technically much more challenging than the printing of polymers or cement, due to the high temperatures involved in metal processing and the tendency for metals to oxidize at elevated temperatures in the presence of air.
Metal parts of complex shapes are commonly needed in the automobile, aerospace, marine, machinery, electronic and medical industries. Machining, casting and deformation processing are conventionally used to fabricate these parts. However, machining produces much waste material and all these methods are inadequate for fabricating some of the complex shapes required. In contrast to machining, 3D printing essentially does not produce waste. This is because 3D printing is an additive process, whereas machining is a subtractive process. Furthermore, 3D printing can produce intricate shapes that these conventional methods cannot produce. The use of 3D metal printing to produce parts (e.g., pipes, fittings, fasteners, etc.) on demand greatly reduces the need to stock spare parts, thus saving storage and facility costs.
The current methods of 3D metal printing mainly include the following methods: (i) laser sintering (more correctly called laser welding) that involves the melting of metal powder by using a laser beam and subsequent solidification of the molten metal upon cooling, (ii) electron beam sintering (more correctly called electron beam welding) that involves the melting of metal powder by using an electron beam and subsequent solidification of the molten metal upon cooling, (iii) gas metal arc welding, which involves the melting of the metal from a wire by using an electric arc and subsequent solidification upon cooling, (iv) liquid metal drop/jet deposition (akin to thermal spraying) and subsequent solidification of the deposited drops, and (v) powder metallurgy (abbreviated PM) (Uetani and Stuber, US20150125334; Deych and Abenaim, US20150115494; Bai et al., US20150069649), e.g., printing a metal particle paste that contains a binder (or successive layers of a binderless metal powder and an organic binder), followed by heating to burn off the binder and subsequent sintering of the metal powder by heating below the melting temperature. The sintering typically takes a long time (e.g., 20 hours). An example of an organic binder is sugar. The binder can be dissolved in a solvent (such as water, alcohol, etc.) and applied as a liquid solution during 3D printing. Alternately, the binder can be in the form of small solid particles that are suspended in a liquid and applied as a suspension. Still alternately, the binder can be dry small solid particles and applied as a powder.
The use of metal powder in the laser sintering, electron beam sintering and PM methods has the disadvantage that the metal powder particles tend to be covered with the native oxide of the metal. For example, aluminum particles are covered by aluminum oxide, which is in the form of an adherent and non-porous coating. (Aluminum foil as commonly used in the kitchen is similarly covered by a protective layer of the native oxide of aluminum.) The removal of the oxide coating on aluminum requires a highly reducing environment. In general, the oxide on the metal particle surface hinders the welding (joining) of the particles. This is because the oxide (e.g., Al2O3) has a much higher melting temperature than the corresponding metal (e.g., Al) and, as a consequence, the oxide does not melt when the metal melts. Furthermore, per unit mass of material, metal powder is expensive compared to metal ingots. Moreover, metal powder that is small in particle size is typically more expensive than the corresponding metal powder with a larger particle size. A sufficiently small particle size is required for the laser sintering, electron beam sintering and PM methods, in order to achieve adequate spatial resolution in the resulting 3D structure and to make the melting or sintering of the particles complete. In addition, due to the large surface area of metal powder compared to a metal ingot with the same mass, metal oxidation (which occurs at the surface) is much more severe for metal powder than metal ingots. The severe oxidation can cause combustion, which is dangerous.
The metal wire used in the metal arc method is expensive compared to a metal ingot with the same mass. The surface area of a metal wire is greater than that of a metal ingot with the same mass. The surface area per unit mass of a metal wire is particularly large when the wire diameter is small. A sufficiently small wire diameter is needed for achieving adequate spatial resolution in the resulting 3D structure and for melting to occur throughout the cross section of the wire. As a result, oxidation (which occurs at the surface) tends to be more severe for a metal wire than a metal ingot with the same mass.
The PM method suffers from the required use of an organic binder. The burn-off of the binder after the printing tends to result in a residue. For example, the burning of sugar (a binder) tends to result in char, which is a carbon solid. The residue is a type of contamination in the resulting product.
Electron beam sintering is expensive (due to the high vacuum required for the electron beam), requires high power and has the printed object size limited by the size of the processing chamber. Laser welding is also expensive, due to the laser. The metal arc method gives a relatively high deposition rate, but it suffers from low accuracy and low resolution, due to the difficulty of controlling the feed of the metal wire into a small liquid pool on the object that is being fabricated. Liquid metal drop/jet deposition suffers from the porosity and rippled surface morphology in the printed metal. In spite of the relatively high printing rate, the PM method is time-consuming, due to the long time needed for the sintering. In addition, powder metal is more expensive and more prone to being oxidized than bulk metal. Furthermore, the sintering associated with PM is conducted in a furnace, so that the size of the 3D printed structure is limited. Conventional 3D metal printing methods such as laser sintering, electron beam sintering, gas metal arc welding and liquid metal drop deposition do not allow fast printing and do not allow large values of the thickness or width of the printed line.
There is a strong need to develop a cost-effective method of 3D metal printing so that the method can provide a high deposition rate without the need for a very high power, as needed for the fabrication of large metal objects. Due to the economic and technical limitations mentioned above, the existing methods of 3D metal printing do not allow the fabrication of large metal objects.
Large metal objects include cars, trucks, railway cars, airframes, aircraft engines, marine vessels, sailing ship masts, street lighting poles, railway tracks, oil well casings, hydroelectric turbines, nuclear reactor control rods, windows, doors, mirrors, astronomical instruments, etc. Small metal objects include car engines, gears, fasteners, watches, cooking utensils, food containers, bicycle components, packaging, outer shells of consumer electronics, heat sinks for electronic appliances, substrates in high brightness light-emitting diode (LED) lighting, etc. The inability to make large objects causes a severe limitation to the range of products that can be made.
Moldless fabrication refers to fabrication without the use of a mold. It is necessary for 3D printing. The absence of a mold allows freeform fabrication, so that various shapes can be achieved.
An organic-based fluid refers to a fluid in which an organic liquid is the base of the fluid, so that the organic liquid is continuous in the fluid. The fluid may contain particles that are inorganic; examples of inorganic particles are metal particles and ceramic particles. However, these particles are not continuous and the fluid remains being based on the organic liquid rather than being based on the particles. Upon curing (e.g., by applying heat or ultraviolet irradiation), the organic liquid (such as a resin) in the fluid is converted to an organic solid (such as a polymer), thereby converting the fluid to a solid (a composite material) that has the organic solid as the matrix (continuous constituent) and the particles as the filler (discontinuous constituent). Such a composite material is to be distinguished from one in which the inorganic constituent is the matrix (continuous constituent). Due to the typically inferior ability of organic materials compared to inorganic materials to resist high temperatures, a composite material with an organic matrix typically has inferior ability to resist high temperatures than a composite material with an inorganic matrix.
In a commonly used method of 3D polymer printing, a thixotropic organic-based fluid is deposited layer-by-layer, thereby resulting in a 3D layered structure. Thixotropy enables the printing fluid (i.e., the fluid before printing) to be shaped by mechanical stress application during printing and furthermore allows the printed fluid (i.e., the fluid after printing) not to sag in the absence of a mold. Sagging means a change in the shape and dimensions, and makes near-net-shape fabrication infeasible.
For 3D metal printing involving the deposition of a fluid in the absence of a mold, a thixotropic metal-based fluid needs to be used. The metal-based fluid has a liquid metal as the base (the continuous constituent) of the fluid. Upon solidification, which occurs during cooling, the fluid becomes a metal-based material (with metal as the continuous constituent). However, the inadequacies of thixotropic metal-based fluids of the prior art (as explained below) make the layer-by-layer moldless deposition method ineffective or infeasible. Thus, instead of thixotropic fluid deposition, the existing methods of 3D metal printing commonly involves the welding of metal particles, using directed heat sources such as lasers and electron beams.
Semi-solid casting is a method of metal casting that involves casting a metal alloy when it is at a temperature between the solidus and liquidus. In this temperature range, the metal alloy is partly liquid and partly solid, i.e., a semi-solid (Withers, US20040261970; Jean-Pierre and Kurt, U.S. Pat. No. 5,186,236; Tetsuichi et al., US20050034837; Tetsuichi et al., US20080127777; Jian et al., US20060038328; Atsushi et al., U.S. Pat. No. 6,860,314; Norville et al., U.S. Pat. No. 6,432,160; Kono, U.S. Pat. No. 5,836,372; Garat and Loue, U.S. Pat. No. 5,630,466; Kirkwood et al., U.S. Pat. No. 5,037,489; Winter et al., U.S. Pat. No. 4,229,210; Buckley, US20030205351; Buckley, U.S. Pat. No. 6,564,856; Leatham and Ogilvy, U.S. Pat. No. 4,804,034; Fan et al., U.S. Pat. No. 6,745,818; Liu et al., US20040137218). In the context of semi-solid casting, a semi-solid refers to a mixture of a solid and a liquid.
A semi-solid typically has the solid in it being non-uniformly distributed. This is commonly due to the unequal densities of the solid and liquid in the semi-solid. If the solid has a higher density than the liquid, the solid would sin. If the solid has a lower density than the liquid, the solid would float. With this non-uniform distribution, the composition is non-uniform upon complete solidification of the semi-solid. The non-uniform composition causes the properties (e.g., the strength and the elastic modulus) to be non-uniform. In addition, the non-uniform distribution of the solid particles in a semi-solid promotes clustering of the particles. Due to the inadequate amount of liquid between the particles in a cluster, the bond between the particles in a cluster is inadequate after the complete solidification of the semi-solid. As a result, a cluster is a mechanical weak region and the presence of one or more clusters weakens the material obtained after complete solidification of the semi-solid.
A semi-solid is not necessarily thixotropic. Its degree of thixotropy is inadequate. As a result, semi-solid casting is inadequate for near-net-shape fabrication (i.e., the solid obtained after complete solidification upon cooling the semi-solid having essentially the same shape and dimensions as the semi-solid before the solidification). Furthermore, the choice of the constituents in a semi-solid is limited and the degree of thixotropy cannot be adequately controlled, as explained below.
A phase refers to a uniform span of matter. For example, at equilibrium at 0 and 1 atm pressure, H 2O consists of two phases, which are solid ice and liquid water. However, at equilibrium at −5 and 1 atm pressure, H 2O consists of a single phase, which is solid ice. Thus, what phases are present depends on the combination of temperature and pressure in the H2O unary (one-component) system.
A phase diagram is a map (a presentation scheme) indicating what combination of conditions (temperature, pressure and composition) gives what phase or phases at equilibrium. It is typically plotted as temperature versus composition, with the pressure fixed at 1 atm. A phase diagram is dictated by thermodynamic considerations, such as the minimization of the free energy. In other words, it is dictated by nature for any particular alloy system. Although the phases under non-equilibrium conditions (such as rapid cooling) can differ from those indicated by the phase diagram, the phase diagram provides baseline information.
As an example of an alloy system, consider a hypothetical alloy system with A and B as the components of the system, such that A and B have unlimited solid solubility in one another. The unlimited solid solubility means that A and B dissolve in one another in any proportion, resulting in a solid solution that has composition ranging from pure A (100% A) to pure B (100% B). This solid solution is denoted as α, as shown in FIG. 1, which is the binary (two-component) phase diagram of this system. The copper-nickel binary system is an example of a system that exhibits a phase diagram that is similar to that in FIG. 1.
Consider, for example, an A-B alloy with overall composition Co, as indicated in FIG. 1. At equilibrium at a temperature above the liquidus (the liquidus temperature being denoted TL in FIG. 1), a single phase in the form of an A-B liquid solution (denoted L) with solution composition Co exists. The solidus is denoted Ts in FIG. 1. At equilibrium at a temperature between TL and TS, two phases coexist; these two phases are α (an A-B solid solution) and L. The temperature T1 is a certain temperature between TL and TS, as indicated in FIG. 1. Upon cooling to T1 and equilibrating at this temperature, α has composition Cα and L has composition CL, such that L is richer in A than α, as shown in FIG. 1. As cooling occurs further in the range between T1 and TS, the phases are still α and L, but the compositions of both α and L change, such that both move toward the left in the phase diagram; in other words, the compositions of both α and L become closer to that of pure A. In addition, the relative proportion of α to L increases as the temperature decreases from T1 and TS. Along with the increase in the α proportion is the typical increase in the size of the α particles that coexist with L.
At equilibrium below TS, α exists as a single phase with composition Co, as shown in FIG. 1. Thus, during the process of solidification, which occurs over the temperature range from TL to TS, the composition and proportion of α change, and the particle size of α typically changes as well. In general, the degree of thixotropic character of a semi-solid depends on the solid proportion and solid particle size. As a consequence, the degree of thixotropic character of the semi-solid (α+L) changes as the solidification progresses. Furthermore, the composition of the solid in the semi-solid (Cα at temperature T1) differs from that of the solid obtained after complete solidification (Co).
As another example of an alloy system, consider a hypothetical alloy system with A and B as the components of the system, such that A and B have limited solubility with one another. This limited solubility means that A and B dissolve in one another to a limited degree, resulting in an A-rich solid solution α and a B-rich solid solution β, as shown in FIG. 2, which is the binary phase diagram of this system. The lead-tin binary system is an example of a system that exhibits a phase diagram that is similar to that in FIG. 2.
Consider, for example, an A-B alloy with overall composition Co, as indicated in FIG. 2. At equilibrium at a temperature above the liquidus TL in FIG. 2, a single phase in the form of an A-B liquid solution (L) with solution composition Co exists. The eutectic temperature is denoted TE in FIG. 2. Solidification under equilibrium conditions occurs upon cooling over the temperature range from TL to TE. In this temperature range, two phases exist, namely the A-rich solid solution α and the liquid solution L. At equilibrium at temperature T1, which is in this temperature range, α has composition Cα, while L has composition CL, such that α is richer in A than L. As the temperature decreases from T1 toward TE, both Cα and CL change, such that both α and L become richer in B, in addition to the relative proportions of α and L changing. Furthermore, these changes are typically accompanied by change in the particle size of α. Therefore, the degree of thixotropic character of the semi-solid (α+L) changes during the solidification. After complete solidification, as in the case of equilibration at a temperature below TE, two solid phases (α and β) exist, with α being rich in A and β being rich in B, such that the overall composition (α and β together) is Co. Thus, the composition of the solid in the semi-solid (Cα at temperature T1) differs from those of the solid phases obtained after complete solidification (as in the case of equilibration at a temperature below TE).
As yet another example of an alloy system, consider a hypothetical alloy system with A and B as the components of the system, such that solid A and solid B have no solubility in one another. The zero value of the solid solubility means that solid A is always 100% A (with no B dissolved in it) and solid B is always 100% B (with no A dissolved in it). Hence there is no solid solution. This situation is illustrated in the binary phase diagram in FIG. 3.
Consider, for example, an A-B alloy with overall composition Co, as indicated in FIG. 3. At equilibrium at a temperature above the liquidus TL in FIG. 3, a single phase in the form of an A-B liquid solution (L) with solution composition Co exists. The eutectic temperature is denoted TE in FIG. 3. Solidification under equilibrium conditions occurs upon cooling over the temperature range from TL to TE. In this temperature range, two phases exist, namely the A (solid) and the liquid solution L. At equilibrium at temperature T1, which is in this temperature range, A has composition 100% A, while L has composition CL, such that A is richer in A than L. As the temperature decreases from T1 toward TE, CL changes, such that L become richer in B, in addition to the proportion of A relative to L increasing. Furthermore, these changes are typically accompanied by change in the particle size of A. Therefore, the degree of thixotropic character of the semi-solid (A+L) changes during the solidification. After complete solidification, as in the case of equilibration at a temperature below TE, two solid phases (A and B) exist, such that the overall composition (A and B together) is Co. Thus, the composition of the solid in the semi-solid (100% A at temperature T1) differs from that of the solid phase B obtained after complete solidification.
In the context of this invention, the degree of thixotropy refers to the extent of the ability of a solid-liquid mixture to maintain its shape and dimensions as cooling occurs, with the cooling causing the solid-liquid mixture to become completely solid in the absence of a mold. A strong degree of thixotropy corresponds to a strong ability to maintain the shape and dimensions as this cooling occurs.
In the context of this invention, near-net-shape fabrication refers to fabrication that gives a completely solidified material (i.e., a product) that exhibits essentially the same shape and dimensions of the corresponding material prior to complete solidification (i.e, the material used to make the product). Near net shape fabrication is attractive because it removes the need for the machining of the product. Machining is expensive, particularly if the product is high in elastic modulus and strength. In addition, machining generates waste, as it is a subtractive process.
The degree of thixotropy of a solid-liquid mixture depends on the proportions of the solid and liquid phases in the mixture, as well as depending on the compositions of the solid and liquid phases in the mixture. In relation to FIGS. 1, 2 and 3, due to the change in the proportions and compositions of the solid and liquid phases as the temperature drops within the temperature range in which solid-liquid coexistence occurs, the degree of thixotropy of the solid-liquid mixture (the semi-solid) changes as the cooling occurs. As a result, the solidification process is not adequate in terms of the extent and controllability of the near net shape fabrication.
The degree of thixotropy of a solid-liquid mixture also depends on the distance of separation between the solid particles in the mixture. The larger is the separation, the lower tends to be the degree of thixotropy. In relation to FIGS. 1, 2 and 3, due to the change in the distance between the solid particles in the solid-liquid mixture as the temperature drops within the temperature range in which solid-liquid coexistence occurs, the degree of thixotropy of the solid-liquid mixture changes as the cooling occurs.
The degree of thixotropy of a solid-liquid mixture also depends on the particle size of the solid particles in the mixture. The larger is the size, the lower tends to be the degree of thixotropy. In relation to FIGS. 1, 2 and 3, due to the change in the size of the solid particles in the solid-liquid mixture as the temperature drops within the temperature range in which solid-liquid coexistence occurs, the degree of thixotropy of the solid-liquid mixture changes as the cooling occurs.
The cooling rate affects the temperature range of the solid-liquid coexistence and also affects the proportions of the solid and liquid phases in the mixture at any given temperature in the temperature range of solid-liquid coexistence. In relation to FIGS. 1, 2 and 3, due to these effects of the cooling rate, the cooling rate affects the degree of thixotropy.
In the context of this invention, near-net-shape fabrication specifically refers to fabrication such that the shape and dimensions of the solid-liquid mixture are essentially the same as those of the completely solidified material obtained by the cooling of the solid-liquid mixture. Without a strong and controlled degree of thixotropy of the solid-liquid mixture, the near net shape fabrication cannot be adequately controlled or achieved.
Although the above explanation is in terms of three examples of phase diagrams, the issues generally apply to any alloy system that involves a temperature range in which solid and liquid phases coexist. Such alloy systems are not limited to those illustrated in FIGS. 1, 2 and 3.
Adequate mechanical properties in terms of reasonably high values of both the elastic modulus and the tensile strength are important for structural applications. The elastic modulus describes the stiffness in the elastic deformation regime, i.e., the amount of stress per unit strain in this regime. The tensile strength describes the highest stress the material can withstand before breaking. The elastic deformation regime is relevant to normal structural operation. In other words, normal structural operation should not involve permanent deformation (plastic deformation) or fracture. Both permanent deformation and fracture occur beyond the elastic regime.
The compositions and relative proportions of the phases in an alloy after complete solidification largely dictate the mechanical properties of the alloy. Since the compositions and relative proportions of the phases are governed by the phase diagram, there is little freedom to choose the compositions and relative proportions of the phases after the cooling. In other words, the choice is very restrictive.
An adequately low value of the CTE is important for structural applications that involve elevated temperatures, because the dimensional changes during heating (thermal expansion) or dimensional changes during cooling (thermal contraction) can be disadvantageous in relation to the operation, reliability and durability of the structure. In case that the structure involves deposition (solidification) of a material as a layer on top of the same material that has already fully solidified and cooled, a low CTE value is important for minimizing the thermal stress and hence improving the bond between the two layers. The compositions and relative proportions of the phases in an alloy after complete solidification largely dictate the CTE value of the alloy. Since the compositions and relative proportions of the phases are governed by the phase diagram, there is little freedom to choose the compositions and relative proportions of the phases after the cooling.
Although solid-liquid coexistence is possible in an alloy under appropriate combinations of overall alloy composition and temperature, the choices of overall alloy composition and the solid-to-liquid proportion are both very limited. In addition, the compositions and proportions of the solid and liquid that coexist cannot be independently chosen, as they are dictated by the phase diagram.
Numerous alloys with solid-liquid coexistence (such as the lead-tin solder alloys) have melting temperatures that are too low or mechanical properties that are too poor for them to be used in practice for structures such as cars. In addition, the lead in the lead-tin alloy is poisonous and is thus an environmental hazard.
A composite material is a material obtained by the artificial combination of different component materials. An example is a composite material obtained by the artificial combination of carbon fibers and a polymer (e.g., epoxy). In this example, there are two components (i.e., carbon fibers and polymer); the polymer is the continuous phase while the carbon fibers are the discontinuous phase. The continuous phase serves as the binder, which serves to bind the fibers together; it is known as the matrix. This composite is an example of a polymer-matrix composite, which refers to a composite that involves a polymer as the matrix (binder).
Another example of a composite material is one obtained by the artificial combination of silicon carbide particles and aluminum. Aluminum is one of the most widely used structural metals. Due to its low density (2.70 g/cm3), aluminum is widely used for lightweight structures such as aircraft structures. Aluminum has a relatively low melting temperature of 660. Silicon carbide (SiC) is a ceramic material that is widely used as an abrasive, due to its high hardness. Silicon carbide has a high melting temperature of 2730. In this example, there are two components (i.e., SiC and aluminum); aluminum is the continuous phase while the SiC particles are the discontinuous phase. The continuous phase is the matrix and serves to bind the SiC particles together. This composite is an example of a metal-matrix composite, which refers to a composite that involves a metal (or metal alloy) as the matrix (binder) (Jin et al., U.S. Pat. No. 5,221,324; Skibo and Schuster, U.S. Pat. No. 4,786,467; Yamamura et al., U.S. Pat. No. 4,622,270; Jatkar et al., U.S. Pat. No. 4,623,388; Choe et al., US20100326739; Morelli et al., US20050085030; Hayashi and Takeda, U.S. Pat. No. 5,372,775; Fox et al., U.S. Pat. No. 6,630,247; Ferrando et al., U.S. Pat. No. 5,858,460; Lhymn and Lhymn, U.S. Pat. No. 5,223,347).
In the context of this Specification, an alloy refers to a metal alloy. An alloy is not a composite material, even if the alloy comprises multiple phases. This is because the phases in an alloy are dictated by nature, in accordance with the combination of temperature, pressure and overall alloy composition. The criteria used by nature involve materials science considerations, such as free energy minimization and phase transformation kinetics. In other words, the phases in an alloy occur naturally and are not present due to an artificial combination of the phases.
The alloy composition refers to the composition of the overall alloy, which includes all the phases in the alloy. The alloy composition is to be distinguished from the phase composition, which refers to the composition of a certain phase.
For example, in the lead-tin binary (two-component) material system, the phases are the lead-rich solid solution, the tin-rich solid solution and the liquid tin-lead solution, such that the phases present and the proportions of coexisting phases depend on the combination of temperature, pressure and alloy composition. Under a certain combination of these conditions, the phases present are the tin-rich solid solution and the lead-rich solid solution, so that the alloy consists of two solid phases; under a certain other combination of conditions, the phases present are the tin-rich solid solution and the liquid tin-lead solution, so that the alloy consists of a solid phase and a liquid phase. The combination of conditions governs not only what phase is present (or what phases are present), it also governs the composition and proportion of each phase present. Each of the phases (the tin-rich solid solution, the lead-rich solid solution and the tin-lead liquid solution) can exhibit a range of composition, with the range depending on the temperature and pressure.
A metal-matrix composite is a composite material with a metal as the continuous phase (the matrix) and a filler as the discontinuous phase. The metal and filler are, in general, different in density and composition.
A method of metal-matrix composite fabrication is stir casting, which refers to a method in which the filler is added to the liquid metal and the mixture is stirred. The mixture is then cast into a mold in which the mixture solidifies to form the metal-matrix composite. In case that the composite is made by stir casting, the filler is non-uniformly distributed, due to the difference in density between the filler and the liquid metal. If the filler has a higher density than the liquid metal, it would sink. If the filler has a lower density than the liquid metal, it would float. Stirring helps to increase the degree of uniformity in the filler distribution. However, stirring has to stop before the start of solidification, and the non-uniformity problem returns once the stirring is stopped. The non-uniformity in the filler distribution after solidification causes non-uniform distribution of the properties (such as the strength and the elastic modulus) in the resulting metal-matrix composite. In addition, the non-uniformity in the filler distribution prior to solidification promotes clumping of the filler units (such as the filler particles). The clumping is not desirable, because the bond between the filler units in a clump is relatively poor, thus causing the mechanical properties of the overall composite to be inadequate.
Another shortcoming of the stir casting method of metal-matrix composite fabrication is that the filler volume fraction is limited to low values, as the viscosity of the mixture of liquid metal and filler increases with the filler volume fraction and stirring is increasingly difficult as the viscosity increases. The low filler volume fraction makes it difficult for the mixture to exhibit thixotropic behavior.
Another method of metal-matrix composite fabrication is powder metallurgy. In this method, a mixture of metal matrix powder and the filler is subjected to pressure and heat so as to consolidate the mixture and cause the metal matrix powder to diffuse in the solid state. This diffusion causes the metal matrix powder particles to bond to one another and to the dispersed filler, thereby forming a monolith, which is the metal-matrix composite. This process is also known as sintering. Powder metallurgy is a solid-state process, without the need for melting the metal matrix material. Thus, no fluid is involved. However, because it is a solid-state process, it is slow and takes a long time.
A short coming of the powder metallurgy method of metal-matrix composite fabrication is that the volume fraction of the filler is limited to low values. This limitation is because a high filler volume fraction corresponds to a low metal matrix volume fraction and adequate bonding between the metal matrix and the filler requires an adequate volume fraction of the metal matrix, which is the constituent that is responsible for the bonding.
Another shortcoming of the powder metallurgy method of metal-matrix composite fabrication is that the size of the composite article is limited by the required high pressure in the fabrication process. Since the pressure is the force divided by the area on which the force is applied, a large area would require a high force in order to achieve the required pressure. The higher is the force, the more expensive is the press (equipment) involved. Therefore, this method can be used for making small objects such as gears, but it cannot be used for the fabrication of large objects such as cars.
Yet another method of metal-matrix composite fabrication is liquid metal infiltration (also known as squeeze casting). In this method, liquid metal is squeezed into the empty space among the filler units (e.g., the filler particles) in a filler preform. The preform refers to a porous object that mainly consists of the filler units, with the pores (empty space) in the preform being among the filler units. Immediately after the infiltration, cooling occurs, so that the liquid metal solidifies, thus resulting in a metal-matrix composite. This method involves the flow of the liquid metal. In other words, the fluid is the liquid metal, which is not thixotropic. The filler units are stationary throughout the process, so that the shape and dimensions of the resulting metal-matrix composite are essentially the same as those of the preform. By not distorting the shape of the preform during the liquid metal infiltration, near-net-shape fabrication is achieved. In order to avoid distortion of the shape of the preform, movement of the filler units in the preform should be avoided. For the purpose of avoiding substantial movement of the filler units, a small amount of a binder (e.g., colloidal silica) is commonly incorporated in the preform during preform preparation. The binder causes the filler units to be essentially locked in their positions, so that the filler units cannot move relative to one another. Instead of using a binder, the preform can be sintered, so that the filler units are bonded together at their points of contact by solid-state diffusion and thus cannot move relative to one another.
A shortcoming of the liquid metal infiltration method of metal-matrix composite fabrication is that the size of the composite is limited by the size of the preform. The size of the preform is in turn limited by the size of the container used for the infiltration process. Therefore, the size of the composite articles made with this technique is limited. This method can be used for making small objects such as gears, but it cannot be used for the fabrication of large objects such as cars.
Still another method of metal-matrix composite fabrication is thermal spraying. In this method, a metal in a powder or wire form is melted into tiny droplets using a heat source (such as a flame). These droplets, mixed with a particulate filler, are sprayed onto a substrate at a high velocity. Due to the high speed of solidification of the droplets, which arrive at the substrate one after another, the solidified material (i.e., the resulting metal-matrix composite) has a relatively high level of porosity. The porosity is detrimental to the mechanical properties. Therefore, the solidified material needs to be subjected to annealing, so as to reduce the porosity. The annealing takes a long time.
Yet another method of metal-matrix composite fabrication is a variation of semi-solid casting that involves casting a mixture of a semi-solid and a filler. The filler is commonly in the form of particles. The semi-solid is an alloy that consists of solid and liquid phases that coexist. The solid phase in the semi-solid is to be distinguished from the filler. The semi-solid may provide a degree of thixotropy to the mixture, if the filler volume fraction is low. However, the degree of thixotropy of the mixture is low, if any.
The filler in a metal-matrix composite is commonly a discontinuous phase, such as particles and short fibers. (Continuous fibers can also serve as the filler, but their incorporation in a composite material cannot involve the mechanical mixing of the fibers and the matrix constituent, as such mixing would make the fibers not straight and may even break the fibers.)
Creep refers to the permanent deformation of a solid due to the temperature being high enough for the solid to exhibit a degree of viscous character. For a metal (not a metal-matrix composite), creep typically occurs at temperatures above about ⅓ of the melting temperature of the metal in Kelvin (not Centigrade). Thus, the higher is the melting temperature of a metal, the greater is the creep resistance of the metal. Creep becomes more severe in the presence of long-term exposure to stress, which can be below the yield strength of the material.
An alloy (to be distinguished from a metal-matrix composite) can be high in creep resistance only if it has a very high melting temperature. Examples of metals with high melting temperatures are tungsten (which has melting temperature 3422) and molybdenum (which has melting temperature 2623). Metals with high values of the melting temperature are known as refractory metals. Casting is a process that involves melting. The high melting temperature causes the casting to require high temperatures, so that the casting consumes much energy and becomes an impractical processing method.
The refractory metals also suffer from the high values of the density. For example, the density of tungsten is 19.25 g/cm3, and the density of molybdenum is 10.28 g/cm3. In contrast, the density of aluminum is only 2.70 g/cm3. The preference for low density (as needed for lightweight structures) limits the choice of alloys.
For 3D metal printing involving layer-by-layer deposition, it is necessary for the creep resistance of the printed material to be sufficiently high. Otherwise, the solidified printed material undergoes creep (dimensional change), particularly before it has cooled sufficiently. Because the dimensions of the printed material should be nearly the same as that of the thixotropic printing material, creep is not desirable. Except for the refractory metals, alloys have low creep resistance. The requirement of high creep resistance limits the choice of alloys.
For 3D metal printing involving layer-by-layer deposition, it is necessary for the CTE of the printed material to be sufficiently low. Otherwise, the thermal stress resulting from the thermal contraction of one layer on top of a layer that has already cooled (already undergone thermal contraction) would weaken the interface between the layers, thus resulting in inadequate bonding between the layers and poor mechanical properties for the resulting 3D structure. Except for the refractory metals, alloys have high CTE values. The requirement of low CTE further limits the choice of alloys.
A metal laminate refers to a plurality of metal layers that are bonded together. For example, aluminum laminates are obtained by the diffusion bonding of aluminum layers together under heat and pressure (Faruque et al., US20140335368). As pressure can cause a degree of deformation of the metal, the pressure application affects the thickness of the laminate. Furthermore, since pressure is equal to the force divided by the area on which the force is applied, the force required for achieving a given pressure is high if the area is large. As a result, the area of the laminate that can be fabricated by diffusion bonding is limited.
The present invention is directed to overcoming these and other deficiencies in the art.